The invention discloses a recombinant strain for expressing beta-galactosidase and a construction method and application thereof. A thermus thermophilus HB27 galactosidase gene bgal is obtained by cloning, inserted into a eukaryotic expression vector pPIC9K and converted to a pichia pastoris strain GS 115, and the expression level of the beta-galactosidase reaches 14,000U/ml; molecular weight expressed by the beta-galactosidase gene bgal is 580Dk, the optimal temperature is 70 DEG C, and the gene is stable at the optimal temperature; the galactosidase has highest hydrolysis activity and high stability when pH is 6.5; the efficiently expressed beta-galactosidase can be directly used for the pasteurization of dairy products, can degrade lactose and sterilize the dairy products simultaneously, and avoids secondary pollution caused by enzyme addition after sterilization; and the HB27 galactosidase can degrade 90 percent of lactose in milk at the temperature of 60 DEG C within 30 minutes.